When Will Neptune Complete Its First Orbit Since Discovery?

As we approach the 164th anniversary of Neptune’s discovery, there’s another reason why we should celebrate this event in astronomical history: the gas giant is less than a year away from completing its first orbit since it was first spotted.

Although this is pretty exciting, the lead-up to Neptune’s first sighting reads like a detective novel, a testament to the ingenuity of astronomers in the 18th and 19th Centuries.

In 1781, British astronomer Sir William Herschel was the first to notice something strange about Uranus’ orbit. By 1821, French astronomer Alexis Bouvard surmised that Uranus was being perturbed by the gravity of another massive planet in the outer solar system. There had to be something out there tugging at the 7th planet from the sun.

Then in the 1840′s, English and French astronomers John Couch Adams and Urbain Le Verrier independently went on to calculate where this mystery planet should be in the night sky by purely measuring these little ‘wobbles’ in Uranus’ path.

55 years after Herschel noticed Uranus’ perturbations, the distant planet was officially discovered by German astronomer Johann Galle in the location predicted by Couch Adams and Le Verrier. It was named Neptune.

As Neptune is located so far away from the sun (approximately 4.5 billion kilometers, 30 Astronomical Units (AU), or 30-times the sun-Earth distance), it takes over 164 Earth years to complete one full orbit around our star.

As the first direct observation of the blue-green gas giant was made on Sept. 23, 1846, Neptune will arrive back in approximately the same spot as where it was first spotted on July 12, 2011.

Neptune was also observed just after it had reached opposition with the Earth — a time when the sun, Earth and Neptune are roughly in alignment and when Neptune makes its closest approach to our planet.

Although Neptune is oblivious of this special time in its orbit, next year will be a special year for astronomy. It will be the first time for nearly 150 years that a planet has completed its first full orbit after its discovery.

Uranus, a planet discovered by Herschel in 1781 — approximately 10 AU closer to the sun than Neptune — completed its first orbit after discovery in the year 1865 (it completes one orbit of the sun every 84 years). And Pluto, the newly designated dwarf planet discovered by U.S. astronomer Clyde Tombaugh in 1930 — approximately 10 AU further away from the sun than Neptune — won’t complete its first orbit for another 168 years. We’ll have to wait until 2178 to see Pluto complete its first 248 year orbit around the sun.

So, enjoy this special time in Neptunian history, we’ll be waiting a while until Pluto joins the party.